Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) in a transmit mode and/or convert received acoustic waves to electrical signals in a receive mode. Acoustic transducers generally include acoustic resonators, such as thin film bulk acoustic resonators (FBARs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. FBAR devices, in particular, generate laterally propagating acoustic waves when stimulated by an applied time-varying electric field confined to finite-sized electrodes, as well as higher order harmonic mixing products. The lateral modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
A stacked bulk acoustic resonator (SBAR), also referred to as a double bulk acoustic resonator (DBAR), includes two layers of piezoelectric materials between three electrodes in a single stack, forming a single resonant cavity. That is, a first layer of piezoelectric material is formed between a first (bottom) electrode and a second (middle) electrode, and a second layer of piezoelectric material is formed between the second (middle) electrode and a third (top) electrode. Generally, the stacked bulk acoustic resonator device allows reduction of the area of a single bulk acoustic resonator device by about half.
In FBAR devices, mitigation of acoustic losses at the boundaries and the resultant mode confinement in the active region of the FBAR (the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode) has been effected through various methods. Notably, frames are provided along one or more sides of the FBARs. The frames create an acoustic impedance mismatch that reduces losses by reflecting desired modes back to the active area of the resonator, thus improving the confinement of desired modes within the active region of the FBAR and minimizing conversion of these modes at the edges of the electrodes into unwanted modes that cannot couple back to electric field (like shear and flexural modes).
While the incorporation of frames has resulted in improved mode confinement and attendant improvement in the quality (Q) factor of the FBAR, direct application of known frame elements has not resulted in significant improvement in mode confinement and Q of known DBARs.
What is needed, therefore, is a DBAR that overcomes at least the known shortcomings described above.